1. Field of the Invention
The invention relates to a cooking stove having a cook top, or hob, for cooking food, and method for production and components thereof. The stove has a stove body, a smooth-top cook top with a coking surface to cook food thereon, and a heat source disposed adjacent to the cooking surface
2. Background Information
Smooth-top glass ceramic cook top cooking surfaces of smooth-top kitchen ceramic or glass ceramic cook tops or a stove having a ceramic or glass ceramic cook top cooking surface have gained considerable popularity as kitchen appliances.
Thus, cooking appliances having ceramic or glass ceramic cooking surfaces are known.
They provide a substantially smooth upper surface on which the various utensils that are to be heated can be disposed.
In these appliances, the cooking zones can be heated, as a rule, by means of electrically operated or gas operated heating devices arranged below the ceramic or glass ceramic cooking surface in the region of the cooking zones. These devices can be, for example, electrically operated contact-heating or radiant heating elements or else gas-jet burners.
An example of a cook top comprises an induction heating apparatus having a cook top including a plurality of induction surface heating units. The cook top comprises a horizontally disposed planar metal support surface having a plurality of openings therein. A ceramic smooth-top plate is supported in each of the openings and adapted to support a cooking utensil thereon. An induction heating coil is supported subjacent to the ceramic plate in a position to generate a magnetic field which passes through the plate to link the cooking utensil. Each plate is supported by a metallic trim frame, which abuts a conductive layer on the plate, with the frame and layer combining to provide a low reluctance flux path, the low reluctance path operating to reduce the magnetic flux leaked into the space surrounding the heating apparatus during operation thereof.
Another example of a cook top has a heating unit that includes two tubular tungsten-halogen lamps, each having a tungsten filament. The lamps are supported within a ring of ceramic fibre material and the unit is preferably mounted beneath an infra-red-transmissive cook top to define a hotplate area of a cooking hob. A control circuit provides a range of discrete power outputs of the lamps, each power output corresponding to a power control setting set by a user of the cooking hob. The circuit includes a phase control circuit for switching power to the lamps at a predetermined phase angle to achieve one or more of the lower power outputs.
Yet another example of a cook top comprises a burner for a “sealed top” range which has a generally upwardly diverging conical body with radially disposed fuel ports and a generally flat removable cap disposed on the upper periphery of the body.
Glass ceramics containing high-quartz solid solutions as the predominant crystal phase are known. A key property of these glass ceramics is that they can be used to produce the materials which have extremely low coefficients of thermal expansion in a predetermined temperature range. As a rule, the thermal expansion behavior is established so that the materials have zero thermal expansion in the range of their temperatures of use. Thus, for example when used as substrate material, wafer stages or mirror supports for telescopes, the thermal expansion is minimized in the region of room temperature. When used as a transparent chimney inspection window or darkened hobs, the zero thermal expansion in a temperature range between room temperature and about seven hundred degrees Celsius is adjusted to as low values as possible.
In transparent form, for example when used as fireproof glass, chimney inspection window or cooking utensils, as a rule high transparency, preferably light transmission of more than eighty percent in the visible range and a defined color location are desired. When used as a hob, a dark coloration which conceals the technical installation beneath the hob is desired. Transmission behavior that makes it possible readily to detect the heating elements during operation, even at low power, while they should remain concealed by the hob in the unused state, is desired. On the other hand, the eyes must not be dazzled or endangered by harmful radiation at high heating powers, in particular with the use of bright halogen lamps. In the infrared radiation range, the transmittance should reach as high values as possible so that the heat radiation can act directly on the face of the pot in order thus to improve the regulation and the initial cooking speed. Thus, a combination of defined high absorption in the visible range combined with low infrared radiation absorption is desired. These requirements are met for a four millimeters thick sample having a light transmittance, measured according to DIN 5033, of tau less than five percent in the visible range and an infrared radiation transmittance at sixteen hundred nanometers of more than sixty-five percent. The term DIN 5033 refers to standard sheet No. 5033 of the German Standards Institute, Deutsches Institut für Normung. DIN 5033 is hereby incorporated by reference as if set forth in its entirety herein.
In the industrial production of glass ceramics, arsenic oxide and/or antimony oxide are used as refining agents. These refining agents are compatible with the required glass ceramic properties and lead to good bubble qualities or low bubble counts in the melt. Even if these substances are firmly bound in the glass skeleton, they are still disadvantageous from safety and environmental protection points of view because particular precautions have to be taken during raw material procurement, preparation and during melting, owing to evaporation, and in subsequent processing procedures. In the disposal of spent glass ceramic products and the dusts and sludges from the production, the arsenic or antimony content adversely affects the recyclability and possibility of disposal in landfills. These substances are often undesired in recycling. Owing to their large surface area, and because of the limits with respect to the sluggishness of arsenic or antimony, dusts and sludges as a rule may be disposed of only on landfills for special wastes.
It is known that the production of glass ceramic products takes place in various stages. After the melting and hot shaping, the material is usually cooled below the transformation temperature of the glass. The starting glass is then converted into the glass ceramic article by controlled crystallization. This ceramization takes place in a two-stage temperature process in which first nuclei, usually comprising zirconium titanate-containing solid solutions, are produced by nucleation at a temperature between six hundred degrees Celsius and eight hundred degrees Celsius. When the-temperature is subsequently increased, the high-quartz solid solutions grow on these nuclei at the crystallization temperature.
It must be ensured by means of the glass ceramic composition that no undesired crystallization (devitrification) occurs during the hot shaping of the glass ceramic particles and, on the other hand, good and controllable crystallization behavior with acceptable process times is achieved in the subsequent ceramization. In many shaping processes, for example also in the rolling of sheets to be used as hobs, the shaping takes place in the proximity of the processing temperature VA of the glass (viscosity eta being equal to ten to the power of four dPa second). For the devitrification behavior, it must be ensured that the upper devitrification temperature of the melt is not above the processing temperature. Otherwise, undesired crystals in the glass can scarcely be avoided. Owing to their size and the growth during the ceramization to even larger crystals, the devitrification adversely affects the strength of the glass ceramic article. In the case of particularly large crystals, these may even be visible, particularly in transparent glass ceramics.
In addition to the stated key requirements with respect to glass ceramics, based on high-quartz solid solutions as the predominant crystal phase, such as, for example, low thermal expansion in the relevant range of use, transparency or possibility of imparting dark coloration, there is a number of further important requirements depending on the respective use. Thus, during prolonged use at high temperatures, such as, for example, chimney inspection windows or hobs, a high temperature/time load capacity is required. The low expansion coefficient responsible for good thermal shock behavior must not change in an impermissible manner under thermal loading during use. Changes in the microstructure which occur with thermal loading during use in practice, in combination with dimensional changes (compaction), must not lead to local tensile stresses and associated impermissible reductions in strength. This phenomenon is particularly critical in the case of hobs where thermally loaded regions (the cooking zones) are adjacent to regions which remain substantially at room temperature. In this boundary region, there must be no impermissibly high compaction stresses. In many applications, the chemical resistance of the glass ceramic articles has to meet high requirements. Chimney inspection windows are often in direct contact with sulfur-containing exhaust gases; in applications as hobs, acid-containing components, for example when food components are overcooked or with the use of acid-containing household cleaners, act on the hob, giving rise to an additional load in the range of high temperatures. In the case of use as a hob, it is furthermore disadvantageous with respect to the temperature/time load capacity if the regions of the cooking zones change with respect to their transmittance with thermal loading during use. With this effect, also referred to as “subsequent darkening”, the temperature/time loading leads to a further reduction in the transmittance in the region of the hot cooking zone and hence to troublesome color differences between cooking zones and cold regions of the hob.
For applications where the very low or zero thermal expansion is not important but where the level of the thermal load capacity is paramount, it should be possible to transform the glass ceramic preferably containing high-quartz solid solutions into glass ceramic containing keatite solid solution. This transformation is effected in acceptable process times in a temperature range of about nine hundred degrees Celsius to twelve hundred degrees Celsius. The glass ceramics preferably containing keatite solid solutions have a coefficient of thermal expansion of the order of magnitude of about one millionths per Kelvin between room temperature and seven hundred degrees Celsius. As a rule, glass ceramics comprising keatite solid solution as the main phase have a translucent or white hue. On addition of colored oxides, the white hue is overcolored according to the coloring effect of the colored oxide.
Known glass ceramics which permit coloring with vanadium oxide and have led to industrially produced glass ceramic products are refined with arsenic and/or antimony oxide.
European patent specification No. EP 0437228 A1, corresponding to U.S. Pat. No. 5,070,045 issued to Comte et al. on Dec. 3, 1991, describes a glass ceramic containing high-quartz solid solutions as the predominant crystal phase, which can be transformed into a white opaque glass ceramic containing keatite solid solutions, the composition necessarily containing arsenic oxide and antimony oxide (arsenic trioxide plus antimony trioxide being equal to five tenths percent to one and five tenths percent by weight). European patent specification No. EP 0437228 A1, and its corresponding U.S. Pat. No. 5,070,045 issued to Comte et al. on Dec. 3, 1991, are hereby incorporated, by reference as if set forth in their entirety herein.
European patent specification No. EP 0220333 B1, corresponding to U.S. Pat. No. 5,212,122 issued to Pannhorst et al. on May 18, 1993, likewise describes a glass ceramic which necessarily contains antimony and/or arsenic oxide (antimony trioxide plus arsenic trioxide being, equal to five tenths to two and one-half percent by weight). European patent specification No. EP 0220333 B1, and its corresponding U.S. Pat. No. 5,212,122 issued to Pannhorst et al. on May 18, 1993 are hereby incorporated by reference as if set forth in their entirety herein.
European patent specification No. EP 0156479 B1 issued to The English Electric Company Limited on Apr. 12, 1989, describes a method for refining a molten lithium aluminosilicate glass using the refining agent cerium dioxide or cerate compounds. The glasses described are free of arsenic and antimony but the colorability with vanadium oxide is not sufficient. Even at comparatively high vanadium pentoxide contents of equal or greater than five tenths percent by weight, a very high transmittance of equal or greater than twenty-three percent is measured at six hundred thirty nanometers. The described high coefficients of thermal expansion of four and nine tenths to nine and five tenths ten millionths per Kelvin between twenty and seven hundred degrees Celsius are also disadvantageous for use as a darkened hob. European patent specification No. EP 0156479 B1 issued to The English Electric Company Limited on Apr. 12, 1989 is hereby incorporated by reference as if set forth in its entirety herein.
It is known that tin dioxide can be used as a nucleating agent in glass ceramics. This is used to reduce the content of the nucleating agent titanium dioxide.
It is thus possible to obtain transparent glass ceramics which have very little natural color owing to a low content of troublesome iron/titanium complex. Thus, Japanese published patent application No. JP 09169542 A, of Nippon Sheet Glass Co Ltd, published on Jun. 30, 1997, describes a transparent glass ceramic containing high-quartz solid solutions as the predominant crystal phase and having a composition which contains zero to one percent by weight of titanium dioxide and one to four percent by weight of tin dioxide. In order to achieve high transparency, arsenic oxide is used as a refining agent. The high tin dioxide contents of equal or greater than one percent by weight adversely affect the devitrification behavior. Japanese published patent application No. JP 09169542 A, of Nippon Sheet Glass Co Ltd, published on Jun. 30, 1997, is hereby incorporated by reference as if set forth in its entirety herein.